Capacitive sensors, also referred to as capacitive transducers, respond to a physical quantity to be measured by a change of a capacitance of the sensor, which in turn may be detected by appropriate readout circuits. Examples of such capacitive sensors include pressure sensors or microphones. Recently, such capacitive sensors have increasingly been implemented as micro-electromechanical systems (MEMS), where the sensor itself together with additional circuitry may for example be implemented on a single silicon chip die. An implementation with two or more separate dies (e.g. one with a mechanical sensor and one with electronic circuitry). In such a case, the separated dies may e.g. be wire-bonded or connected with other techniques like through silicon via (TSV).
Such capacitive sensors typically include a movable membrane and one or more back plates, capacitances being formed between the membrane and the one or more back plates. The physical quantity to be measured, for example pressure or sound, causes the membrane to move, thus changing a capacitance between the membrane and the one or more back plates. In differential implementations, the membrane is arranged between two back plates, forming two capacitances. When the membrane moves, one of the capacitances (between the membrane and one of the back plates) increases, while another capacitance (formed between the membrane and the other one of the back plates) decreases. For reading out the sensor, a bias is applied to the capacitances. A change in the capacitances then causes an output signal.
Various general approaches exist for reading such capacitive sensors. A first approach is referred to as constant voltage biasing herein. In this approach, a bias voltage is kept constant, and change of capacitances cause a current to flow, which serves as the output signal. Another approach is referred to as constant charge biasing herein. Here, the capacitances are biased via a high impedance connection. When the capacitances change, the charge on the capacitances remains essentially constant at least on typical timescales of the capacitance variations, as due to the high impedance charge cannot leave the capacitances fast enough. This causes a generation of a voltage signal, which may be read out via high impedance amplifiers.
When manufacturing such capacitive sensors, capacitances may vary for example due to process variations. Furthermore, in case of differential sensors the capacitances, which in a rest position of the membrane mentioned above may have nominally equal values, may in fact have different values due to such process variations. For these and other reasons, it is desirable to test the sensor, for example to measure capacitances or a difference between capacitances.
Various approaches have been made for such measurements, which generally involve the application of a test signal to the sensor. While many of these approaches are quite suitable for sensors being used in constant voltage biasing schemes, they may not be fully suitable for constant charge biasing schemes. In particular, in constant charge biasing schemes the high impedances used, which may be in the GΩ range or higher, in practice act as filters with a comparatively slow time constant, which makes application of suitable test signals without disturbing the actual operation of the sensor difficult.